Fluoride-Free Siloxane Circularity at Room Temperature: Tetrabutylammonium Hydroxide as a Dual-Functional De- and Repolymerization Initiator for Silicone Oils, Elastomers, and Hybrid Coatings

Abstract With increasing EU restrictions on fluoride chemistries and their toxicity related to the potential generation of hydrofluoric acid (HF) as well as their corrosivity, there is an urgent need to develop fluoride-free depolymerization methods to enable safe and sustainable siloxane recycling. In this regard, we here present a fluoride-free, at room-temperature (RT) operating depolymerization method, where tetrabutylammonium hydroxide (TBAOH), identified as a highly effective dual-functional de- and repolymerization initiator, transforms silicone-based elastomers into reusable cyclic siloxanes. Thereby, the solvent/polymer ratio (v/w) is the key for controlling the product distribution: at 10/1 (v/w), the depolymerization mixture shifts from D4-rich to high proportions of byproducts greater than 40 mol % and ultimately stops the process, leaving solid educt silicone at even lower dilutions. Under optimized reaction conditions above a threshold of 0.29/1 (w/w) for the TBAOH/polymer ratio, a relative proportion of up to 88 mol % cyclic siloxanes (D3 to D5), with D4 as the main product, is obtained. This corresponds to 77 mol % absolute NMR conversion into cyclic siloxanes. Additionally, it is worth noting that the fluoride-free, RT operating reaction conditions are effective for a very broad educt scope ranging from linear polydimethylsiloxane (PDMS) oils, (filled) Pt-cross-linked silicone elastomers, to inorganic–organic hybrid polymers and coatings. Compared to sodium hydroxide (NaOH) as an alkaline reference, TBAOH affords fewer noncyclic byproducts (12–26 mol % vs 25–30 mol %) and a higher D3 (4–14 mol % vs 1–3 mol %) relative proportion, highlighting its distinct reactivity. Fluoride-free siloxane circularity is demonstrated at RT by two complementary repolymerization routes. Within the first method, TBAOH uniquely acts as the initiator for both the depolymerization and the subsequent anionic ring-opening polymerization (AROP) without prior workup, while the second involves the isolation of cyclic siloxanes followed by base-catalyzed AROP. Thus, this TBAOH-enabled, fluoride-free depolymerization method, which operates at RT, provides tunable product distributions and a broad substrate applicability for practical silicone circularity.